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Basaltic Volcanism and Tectonics of the High Lava Plains, Southeastern Oregon

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Basaltic Volcanism and Tectonics of the High Lava Plains, Southeastern Oregon AN ABSTRACT OF THE DISSERTATION OF Brennan T. Jordan for the degree of Doctor of Philosophy in Geology presented on June 28, 2001. Title: Basaltic Volcanism and Tectonics of the High Lava Plains, Southeastern Oregon. Redacted for privacy Abstract approved: Redacted for privacy David W. Graham The High Lava Plains province (HLP) of southeastern Oregon is a Miocene to Recent volcanic upland characterized by widespread basaltic volcanism and west-migrating rhyolitic volcanism. New 40Ar/39Ar ages for HLP rhyolites demonstrate that the trend of migrating rhyolitic volcanism is robust, reflecting westward migration at a rate of -35 kmlm.y. from 10 to 5 Ma, and -15 km/m.y after 5 Ma. This pattern mirrors the trend of northeastward migrating silicic volcanism of the Snake River Plain to the Yellowstone Plateau. HLP basaltic volcanism was relatively continuous with episodes of heightened activity at -7.6, -5.9, and 2-3 Ma. The 7.6 Ma event coincided with initiation of High Cascades volcanism suggesting a major regional tectonic event. HLP basalts are variably evolved high-alumina olivine tholeiites. Even primitive basalts are enriched relative to mid-ocean ridge basalts (MORB) in incompatible trace-elements, especially Ba, Sr, and Pb. HLP basalts are isotopically evolved relative to MORB with 87Sr/86Sr of 0.70305 to 0.70508 andENdof +6.7 to + 1.6. Isotopic characteristics of Pliocene and Quaternary basalts are more evolved in the east than the west. Miocene basalts are of more uniform isotopic character. Helium isotopes in Quaternary basalts are constant across the HLP with 3He/4He of -9RA,reflecting either a strongly depleted MORB source or interaction with a mantle plume. The HLP and Snake River Plain are linked by divergent trends of silicic volcanism and a belt of Pliocene and younger basaltic volcanism. To explain both provinces I propose the following. At 17 Ma a small plume head was emplaced under the North American lithosphere, centered near Twin Falls, Idaho, the location predicted by plate tectonic reconstructions. Basaltic volcanism (Columbia River and Steens Basalts) resulted from emplacement of plume head material under thin lithosphere west of the craton margin and from westward flow from the plume up the lithospheric topography at the craton margin. The latter process may also have driven westward mantle flow under the HLP. Westward migrating volcanism of the HLP may also reflect greater times to incubate crustal magmatism further from the center of the plume head. Basaltic Volcanism and Tectonics of the High Lava Plains, Southeastern Oregon By Brennan T. Jordan A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed June 28, 2001 Commencement June 2002 Doctor of Philosophy dissertation of Brennan T. Jordan presented on June 28. 2001 APPROVED: Redacted for privacy Co-Major Professor, repiQ'senting Geology Redacted for privacy Co-Major Professor, representing Geology Redacted for privacy Head of Department of Gêosciences Redacted for privacy Dean of Gpàduàte School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Redacted for privacy Brennan T. JorLlan. Author ACKNOWLEDGMENTS Major funding for the research described in this dissertation was provided by the National Science Foundation by way of a grant to Anita Grunder, Dave Graham, and Bob Duncan. Additional funding was provided by a Student Research grant from the Geological Society of America. The NSF-RIDGE Program funded my participation in the 2000 Iceland Summer School, which contributed greatly to my perspective on plume processes. I thank my advisors, Anita Grunder, Dave Graham, and Bob Duncan, for their participation in this research. Particularly, I appreciate their willingness to take one more shot at pursuing grant-support for this research. I thank Anita for her deep involvement in this project, her critical consideration of the problems posed by High Lava Plains magmatism, and her attempts to keep me on the petrologic straight and narrow. Dave and Bob offered analytical assistance and intellectual contributions well beyond the budgeted support for this project. I also thank Gene Humphreys of the University of Oregon for sharing his ideas on geologic processes of the western United States. Many aspects of my research and studies at Oregon State University benefited from interactions with other faculty in the Department of Geosciences and the College of Oceanic and Atmospheric Sciences, notably, Ed Taylor, Dave Christie, Roger Nielsen, and Andy Ungerer. Special thanks to Lew Hogan and John Huard for their help in the argon geochronology lab. I thank my colleagues at Southern Oregon University, Monty Elliott, Jad D'Allura, Eric Dittmer, Charles Lane, and Joe Graff, for friendship while I taught at SOU and for their unwavering insistence that progress be made on this document while I was there. I also wish to thank the professors who shepherded me though my earlier phases of education in geology: Charles Merguerian (Hofstra University), Karl Karistrom (Northern Arizona), Dave Rodgers (Idaho State University), and Paul Link (Idaho State University). I reflect upon my experiences with them frequently, and am very grateful for their contributions. Lastly, I acknowledge the support of my family. My mother, father, and grandmother strongly supported this endeavor and offered critical financial assistance when needed. I thank my wife, Jeanne, and stepchildren, Caleb and Justine, for their love, support, and patience while I have pursued this degree. CONTRIBUTION OF AUTHORS Anita Grunder, David Graham, Bob Duncan, Alan Demo (Berkeley Geochronology Center), and Bruce Nelson (University of Washington) are listed as co-authors for the manuscripts submitted as chapters 2, 3, and 4 of this dissertation. Grunder collected a significant number of the samples analyzed in this study (those with sample number format HLP-98-##), and assisted in the selection of samples for geochemical and geochronologic analysis. Grunder and her former students Martin Streck, Jim MacLean, and Jenda Johnson collected all of the samples analyzed by 40Ar/39Ar at the Berkeley Geochronology Center. Graham assisted in sample selection for helium isotope analysis, and performed these analyses. Duncan assisted in performing 40Ar/39Ar analyses at Oregon State University and in the interpretation of these results. Demo performed 40Ar/39Ar analyses conducted at the Berkeley Geochronology Center and interpreted the results of these analyses. Nelson performed strontium, neodymium, and lead isotope analyses. Grunder, Graham, and Duncan provided critical reviews of the three manuscripts constituting chapters 2,3, and 4 and suggested substantive revisions. TABLE OF CONTENTS Chapter 1: Introduction to the Problem of High Lava Plains Magma Genesis .1 The High Lava Plains volcanic field............................................................1 Rhyolites of the High Lava Plains....................................................5 Basalts of the High Lava Plains.......................................................6 Structural Geology of the High Lava Plains and Adjacent Northern Basin and Range.........................................................7 Models for High Lava Plains Magmatism.................................................10 Propagating Rifts or Shear Zones...................................................10 Plume Head Entrainment...............................................................11 Evolving Back Arc Extension........................................................14 Introduction to Following Chapters...........................................................16 Chapter 2: Geochronology of Oregon High Lava Plains Volcanism: Mirror Image of the Yellowstone Hotspot?..............................................................17 Abstract.................................................................................................... 18 Introduction.............................................................................................. 19 The High Lava Plains................................................................................22 VolcanicRocks.............................................................................22 Sturcture........................................................................................23 Previous Geochronology...............................................................24 Methods....................................................................................................25 Berkeley Geochronology Center....................................................26 Oregon State University................................................................27 Results......................................................................................................27 TABLE OF CONTENTS (CONTINUED) Discussion................................................................................................. Propagation of Rhyolitic Volcanism..............................................37 Middle Miocene Silicic Volcanism................................................41 Comparison of the HLP and YSRP trends......................................45 Cause of Migrating Volcanism of the HLP....................................48 Conclusions.............................................................................................
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